The present invention relates to fire detectors using an ionization chamber as the essential detection agent.
It is well known to detect fires in a room or a given atmosphere by using an ionization chamber in the manner indicated hereinunder.
The atmosphere in which it is desired to monitor the possible appearance of signs of fire, such as for example smoke, is linked with the atmosphere of an ionization chamber in which the ambient air is ionized under the influence of the (generally .alpha.) radioactivity produced by a radioisotope. In normal operation, this .alpha.-emitting radioisotope causes a relatively stable and constant ionization within the chamber, so that a current is established between the two electrodes of said chamber and the intensity of said current is substantially constant provided that there is no disturbance of the chamber atmosphere. If, however, as a result of an incipient fire, smoke or fumes appear in the atmosphere and penetrate said chamber, said smoke will contain particles having a relatively high concentration and will exert a disturbing influence both on the formation and on the displacement of the ions within the ionization chamber. When said particles are ionized they are less mobile than ions from the ambient air. These heavy ions move much more slowly in the ionization chamber and there is a much greater chance of their recombining with an ion of opposite sign to give a neutral particle than in the case of ions from the ambient air. This increased recombination leads to a drop in the ionization current. In other words, the appearance of the combustion gas in the chamber leads to a significant increase in the apparent electrical resistance thereof. This drop in the ionization current then triggers off the fire alarm signal.
However, different disturbances tend to prevent correct operation of such a detector if a minimum of precautions are not taken. Firstly, the pressure and temperature variations of the external atmosphere can be corrected so that the rest current of the ionization chamber is independent of these two latter parameters. Conventionally, such a correction is obtained by fitting a compensation chamber in opposition with the actual detection chamber.
Although such a process is satisfactory for compensating variations in the pressure and temperature, it is inadequate for preventing the main cause of false alarms which is the condensation of water vapour on the radio isotope source following a sudden drop in the temperature when the atmosphere is very humid. Thus, the mean free path of .alpha. particles in air at ordinary pressure is 3 to 5 cm, whilst it is only 0.2 to 0.03 mm in water. When as a result of water vapour condensation on the source, the latter is covered with a film of liquid water, this leads to an abnormal absorption of the .alpha. radiation, which no longer fulfils its ionization function and consequently there can be large drop in the ionization current and in fact it can almost be eliminated. The detector then behaves as if it was filled with combustion gas and triggers off a false alarm. To obviate this difficulty, it has been proposed (see particularly French Pat. No. 1,185,495) to maintain the radioisotope source at a temperature slightly higher than ambient temperature by means of a heating wire and using the Joule effect. Unfortunately, in the hitherto known constructions, the heating wire was placed in the vicinity of the emitting surface of the source, turned towards the inside of the ionization chamber, which led to two serious disadvantages.
On the one hand, such an arrangement leads to the heating of the ionized medium within the ionization chamber and to the serious modification of the coefficient of mobility of the ions formed, which is directly dependent on the temperature. On the other hand, the interior of the chamber is thus directly subject to the influence of the electrical field created by the passage of the electrical current in the heating wire and this also disturbs the mobility and path of the ions formed.